Sucrose fatty acid esters (hereinafter abbreviated as "SE") are esters formed between sucrose and fatty acids, and their HLB value or other characteristics are subject to variation depending on the degree of substitution on the hydroxyl groups of the sucrose molecule, the carbon atom number of the fatty acid, and so on. They are used chiefly in the field of foodstuffs as emulsifying agents, foaming agents, bacteriostats, or substitutes for edible fats and oils. In particular, sucrose fatty acid esters having an average degree of hydroxyl substitution of 3 or higher, which are lipophilic, have attracted attention as substitutes for edible fats and oils, e.g., spreads, baking fats and oils, and salad oil or as carriers for medical use.
Processes for preparing SE are roughly divided into solvent methods and solvent-free methods. The solvent methods are characterized in that SE can be produced under relatively mild conditions while suppressing formation of by-products due to decomposition of sucrose. The solvent-free methods hold an advantage of simple operation because of no use of any solvent. However, since the ester interchange starts at a high temperature (130.degree. to 160.degree. C.) from the initial stage with the sucrose proportion in the reaction system being high, the reaction involves considerable decomposition of starting materials, such as sucrose. This results in low yield of SE per sucrose and noticeable coloration of the products. In this connection, many proposals on improvement of the solvent-free methods have hitherto been made.
It is known that SE having desired characteristics can be obtained by using other SE species of different characteristics as a part of starting materials according to the solvent-free method. For example, JP-A-59-78200 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a process in which a mixture of an SE, a soap, and sucrose is melted by heating to obtain an SE having a higher HLB value, i.e., a lower degree of substitution, than that of the starting SE. The disclosure suggests to start with an SE having an HLB value of 3 or more, preferably from 5 to 12. In the working examples of the publication, an SE having an HLB value of 14.0 or 10.5 was prepared by starting with an SE having an HLB value of 10.5 or 7.0, respectively. No case has been reported in which an SE of low substitution is obtained from an SE of high substitution having an HLB value of less than 3 (corresponding to an average degree of substitution of from about 3 to 8).
The soap used as a reaction assistant in a solvent-free method generally has a large carbon atom number and is used in a large quantity. In the working examples of JP-A-59-78200 supra., for instance, a soap having 16 or 18 carbon atoms was used. However, since water solubility of a soap decreases as the carbon atom number increases, easy purification techniques such as liquid-liquid extraction cannot be adopted without difficulty. With respect to the amount of the soap, the publication states that the soap is preferably used in a relatively large amount, e.g., from 100 to 150% by weight based on the starting SE. However, because of the difficulty in removing a soap having a large carbon atom number, use of such a large quantity of a soap leads to economical and operational disadvantages in production on an industrial scale.
On the other hand, it has been proposed to replace the soap with an SE having a different HLB value from the starting SE as a melting assistant to obtain an SE having high degree of substitution. For example, JP-A-61-106589 discloses a process comprising reacting sucrose and a fatty acid lower alkyl ester in a high temperature in the presence of a sucrose fatty acid ester having an average degree of substitution of 3 or more as a melting assistant. In the working example of the publication, sucrose, an SE (sucrose stearate), and a fatty acid lower alkyl ester (methyl stearate) were reacted at a high temperature of 160.degree. C. in the presence of potassium carbonate. According to the example, however, the resulting reaction mixture was brown-tinted and found to contain a by-produced long-chain (C.sub.18) soap.
While various manipulations have been proposed for purifying the produced SE, none of them is satisfactory. For example, it has been suggested to treat the reaction product with a strongly alkaline aqueous solution followed by centrifugal separation to remove the soap thereby to control the alkali metal ion level below 1 ppm as disclosed in JP-A-1-207296 and JP-A-1-211594. However, fatty acid esters, inter alia, sucrose fatty acid esters, though stable around neutrality, are susceptible to hydrolysis under alkaline or acidic conditions. Further, the above-described alkali treatment tends to induce coloration (caramelization) due to, for example, release of hydroxyl groups from sucrose molecules, forming colored furan compounds (e.g., furfural, furfuryl alcohol). This requires an additional purification step such as decoloring, resulting in a great economical and operational disadvantage in industrial production.